The Birth of the Dolomites - Beautiful Mountains born out of the Sea

"The surface of the earth is far more beautiful and far more intricate than any lifeless world. Our planet is graced by life and one quality that sets life apart is it's complexity."

Carl Sagan in "Cosmos - The Persistence of Memory"

In July 1791 the French aristocrat, adventurer and naturalist Diedonnè-Silvain-Guy-Tancrede de Gvalet de Dolomieu published a short article describing a peculiar limestone he had observed during a voyage in the Alps. The white rock was very similar to common limestone, but the mineral grains forming the unusual rock showed almost no reaction with acids, unlike crystals of calcite or aragonite (the main minerals of limestone), which react violently. Three years later the naturalist Richard Kirman introduced the Dolomite as a new mineral; the name from there became used to name the dolostone rocks and finally gave the Dolomites - referred in the past simply as the "Pale Mountains" - their actual name.

In the 19th century the genesis of the Dolomite Mountains was one of the great geological mysteries. Fossils provided clues that the rocks composing the mountains were formed once in the sea by living organisms, but in these early days of geology almost nothing was known about the bottom of the sea and the sedimentation occurring in oceans.

Fig.1. Fossils of corals - Margarosmilia sp.

June 11, 1770 the explorer James Cook discovered, not entirely voluntarily (the "HMS Endeavour" collided with it) , one of the largest bioconstructions on planet earth - the Great Barrier Reef of Australia. Here apparently gigantic mountains of limestone were formed below the surface of the sea - but how exactly and how could these mountains raise from the bottom of the sea and form one of the most intriguing landscapes in the world?

One of the earliest descriptions of coral reefs comes from a certain Mr. Strachan, who in 1704 submitted to the Royal Society in London a three pages paper speculating about the formation of these structures:

"There are big banks of this coral, it is porous and so hard or yet as smooth as the upright, which grows in small branches. If, of which we speak, is fully grown, others grow in between it, where still others will grow, until the whole structure is as hard as a rock."

This idea was probably not verified in the field, but based on the few travel accounts brought back from ships venturing in the Indian and Pacific Ocean at these times.

In 1772 to 1775 the German naturalist Georg Forster (1729-1798) acted as naturalist during one of the expeditions of James Cook. They visited the atolls and volcanic islands of the Pacific Ocean. Forster observed that corals live in the first meters of the water column, but that a reef rises up to 300-600 meter above the ocean floor. He developed two hypotheses to explain this observation. The corals grow slowly from the bottom of the sea until reaching the surface, where erosion levels the reef to form the plain surface of an atoll, or in alternative, violent volcanic eruptions pushed patches of corals to the surface.

Almost a half century later another naturalist became intrigued by the mysterious connection between corals and atolls. During his voyage on board of "HMS Beagle" (1831-1836), young geologist C. Darwin studied Lyell's "Principles of Geology" and the chapter about reefs in the Pacific stimulated his imagination. In Chile, February 20, 1835, Darwin had experienced a very strong earthquake and shortly afterward noted evidence of several meters of uplift in the region. In accordance to Lyell's view Darwin imagined that mountains could rise and sink by many similar events during geological time.

Based only on the description in the book of atolls, and assuming slow movements of the surface of earth, Darwin developed a preliminary hypothesis to explain the formation of atolls in the middle of the sea. He admits in his 1887 autobiography:

"No other work of mine was begun in so deductive a spirit as this; for the whole theory was thought out on the west coast of S. America before I had seen a true coral reef. I had therefore only to verify and extend my views by a careful examination of living reefs. But it should be observed that I had during the two previous years been incessantly attending to the effects on the shores of S. America of the intermittent elevation of the land, together with the denudation and deposition of sediment. This necessarily led me to reflect much on the effects of subsidence, and it was easy to replace in imagination the continued deposition of sediment by the upward growth of coral. To do this was to form my theory of the formation of barrier reefs and atolls."

Darwin recognized that the animals forming the corals needed sunlight, so the corals couldn't grow on the dark bottom of the sea. Darwin imagined therefore that the peaks of extinct volcanoes approaching the surface of the sea, a common feature in the oceans he visited, would experience a slow subsidence. These movements were slow enough to enable the corals to compensate the down-movement and keep living on sea level, where plenty of light and nutrients were available.

Fig.3. Atoll formation and reef growth (after DARWIN 1842). Darwin proposed that volcanic islands with fringing reefs, islands with barrier reefs and atolls (i.e. ring-shaped reefs without a volcanic island) are different stages of one process, controlled by time, subsidence of the volcanic core and reef growth. This famous concept is based on surface examination of reefs and comparison of islands and atolls in different stages of development. Data on the slopes and basins were virtually absent at the time.

Darwin's hypothesis was very speculative, based only on superficial observations - there was simply no way to study the shape and base of coral reefs at the time. Nevertheless Lyell inserted Darwin´s theory in later editions of his "Principles" and the American geologist James Dwight Dana (1813-1895), who in 1838-1842 visited the Pacific, confirmed most of the observations of Darwin.

Important modifications to this three-stage reef-theory were added in 1868, when the German zoologist Carl Semper (1832-1893) described on the island of Palau the simultaneous occurrence of the three diverse reef types, contradicting the temporal sequence as proposed by Darwin. In 1878 and 1880 the oceanographer John Murray (1841-1914) published his observation made during the Challenger-Expedition (1872-1876) on the islands of Palau and the Fijis. He postulated that reefs grow on submarine elevations of any kind if they are high enough, not only volcanoes.

This new theory was strongly supported and modified by the geologist Alexander Agassiz and others. Atolls grow up from shallow submarine elevations of various origins. Corals in the middle of the reef will die due the reduced circulation of water, then the calcareous skeleton of the reef building organisms is dissolved by the agents of erosion. In the end a lagoon and the characteristic shape of an atoll forms.

These observations of living reefs in the tropical seas provided new impulses to interpret the geological relationships in the Dolomites. In 1860 the Austrian geologist Baron Ferdinand F. von Richthofen (1833-1905) visited and studied the area of the Dolomites. He discovered that the sandstone and tuff deposits, surrounding the isolate peaks of dolostone, contained large limestone boulders, some containing still recognizable fossils of corals. Based on the theory of evolution of a reef as proposed by Darwin, Richthofen suggested that the isolated peaks were the intact remains of an ancient reef, still surrounded by clastic sediments of an ancient ocean basin, in which, from time to time, landslides from the steep slopes of the reef deposited large boulders of corals.

Fig.4. & 5. The "Richthofen-Riff", a part of a Triassic reef with tongues of former submarine landslides (from the left to the right) into the sediments of a basin (St. Kassian-Fm; Wengen-Fm, mainly sandstones and marls) by MOJSISOVICS 1879.

Fig.6. Closeup of a limestone boulder (former debris coming from the reef), so called Cipit-Kalkblöcke, interbedded in the Wengen-Fm.

The young geologists Edmund Mojsisovics von Mojsvar (1833-1905) developed further this reef hypothesis, mapping in detail the relationships between the single facies (a term describing the appearance of a rock and correlated depositional environment), ranging from the lagoon of the atoll to the open sea. Massive, many thousands of meter thick reefs of dolostone changed suddenly to well bedded carbonates, deposited in a central shallow lagoon. The former slope of the reef was composed of tongues of reef debris interbedded within sandstones, shale and basalts deposited on the bottom of the sea.

Fig.7. Examples of slope bedding in the outer parts of the carbonate platforms of the Dolomites (after MOJSISOVIC 1879). Scheme of bedding on the flanks of carbonate platforms and examples of flank and basin deposits from the Sciliar/Schlern platform. Note the abundant limestone boulders in the basin sediments.

Such strong sedimentary facies changes were until then considered impossible. The reconstruction of the Dolomites as an ancient atoll landscape seemed so radical, that Mojsisovics was obligated to find a private publisher for his revolutionary work (MOJSISOVIC 1879).

Bibliography:

AGASSIZ, A. (1903): Reports on the Scientific Results of the Expedition to the Tropical Pacific, the Marshall Islands. Mem. Mus. Comp. Zool. Harvard College 28: 271-329

DARWIN, C. (1837): On certain areas of elevation and subsidence in the Pacific and Indian oceans, as deduced from the study of coral formations. Proceedings of the Geological Society of London 2: 552-554

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